Apparatus and method for transmitting ACK/NACK messages in a wireless communication system

ABSTRACT

A base station for use in wireless network that communicates with subscriber stations according to a multicarrier protocol. The base station receives an uplink subframe comprising a plurality of blocks. Each of the blocks comprises up to N subcarriers transmitted by the subscriber stations. The base station receives a first acknowledgment signal transmitted by a first subscriber station in the uplink subframe. The first acknowledgment signal is carried on at least one selected subcarrier in a first block of the uplink subframe and on at least one selected subcarrier in a second block of the uplink subframe.

CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY

This application is related to U.S. Provisional Patent No. 60/796,332,filed Apr. 28, 2006, entitled “Hybrid ARQ Feedback In A WirelessCommunication System” and to U.S. Provisional Patent No. 60/795,953,filed Apr. 28, 2006, entitled “Hybrid ARQ ACK/NACK Scheduling In AWireless Communication System”. Provisional Patent Nos. 60/796,332 and60/795,953 are assigned to the assignee of this application and areincorporated by reference as if fully set forth herein. This applicationhereby claims priority under 35 U.S.C. §119(e) to Provisional PatentNos. 60/796,332 and 60/795,953.

This application is related to U.S. patent application Ser. No.11/390,056, entitled “System And Method For Dynamic Allocation Of ARQFeedback In A Multi-Carrier Wireless Network”, filed Mar. 27, 2006.Application Ser. No. 11/390,056 is assigned to the assignee of thisapplication and is hereby incorporated by reference as if fully setforth herein.

TECHNICAL FIELD OF THE INVENTION

The present application relates generally to wireless communicationsand, more specifically, to a technique for transmitting ACK/NACKmessages using frequency diversity in an OFDM or OFDMA network.

BACKGROUND OF THE INVENTION

Orthogonal frequency division multiplexing (OFDM) is a multi-carriertransmission technique in which a user transmits on many orthogonalfrequencies (or subcarriers). The orthogonal subcarriers areindividually modulated and separated in frequency such that they do notinterfere with one another. This provides high spectral efficiency andresistance to multipath effects. An orthogonal frequency divisionmultiple access (OFDMA) system allows some subcarriers to be assigned todifferent users, rather than to a single user. Today, OFDM and OFDMAtechniques are used in both wireline transmission systems and wirelesstransmission systems.

In conventional OFDM/OFDMA networks, a dedicated resource is allocatedto each subscriber station (e.g., mobile device, wireless terminal,etc.) for ARQ feedback or hybrid ARQ feedback, such as an acknowledgment(ACK) message or a negative acknowledgment (NACK) message. By way ofexample, a transmitter (e.g., base station) in a conventional OFDMwireless network sends the data packets along with the controlinformation to a receiver (e.g., subscriber station). The controlchannel carries information specifying, for example, the sequence numberand the modulation and coding scheme used to encode the data packet. Thesubscriber station attempts to decode the data packet and then, in thededicated ACK/NACK channel, transmits to the base station a feedbackmessage regarding either a successful or an unsuccessful transmission.

In conventional systems, the ACK/NACK signal is time-multiplexed withina subframe of the uplink or reverse channel (i.e., from subscriberstation to base station) using a very short transmission duration.However, an ACK/NACK signal transmitted for only a short durationcarries only a small amount of energy. This results in very high biterror rate (BER) in the ACK/NACK channel, thereby resulting in limitedsystem throughput and coverage.

Therefore, there is a need for improved OFDM or OFDMA transmissionsystems that maximize throughput and coverage. In particular, there is aneed for improved OFDM or OFDMA transmission systems that transmitACK/NACK signals in the uplink with a lower bit error rate.

SUMMARY OF THE INVENTION

In one embodiment of the disclosure, a base station is provided for usein wireless network capable of communicating with a plurality ofsubscriber stations according to a multicarrier protocol. The basestation receives an uplink subframe comprising a plurality of blocks.Each of the blocks comprises up to N subcarriers transmitted by theplurality of subscriber stations. The base station receives a firstacknowledgment signal transmitted by a first subscriber station in theuplink subframe. The first acknowledgment signal is carried on at leastone selected subcarrier in a first block of the uplink subframe and onat least one selected subcarrier in a second block of the uplinksubframe.

In another embodiment, a first communication device is provided thattransmits a first message in a forward channel to a second communicationdevice and receives multicarrier signals in a reverse channel from othercommunication devices. The reverse channel comprises a plurality oftransmission slots, where each of the transmission slots comprises up toN subcarriers transmitted by the other communication devices. The firstcommunication device receives in the reverse channel an acknowledgmentsignal transmitted by the second communication device in response to thefirst message. The acknowledgment signal is received on a first selectedsubcarrier in a first time slot of the reverse channel and on a secondselected subcarrier in a second time slot of the reverse channel.

In still another embodiment, a base station is provided for use in awireless network that communicates with a plurality of subscriberstations according to an orthogonal frequency multiple access (OFDMA)protocol. The base station transmits a message to a first subscriberstation in a downlink channel and receives in an uplink channel anuplink subframe comprising OFDMA subcarriers transmitted by theplurality of subscriber stations. The uplink subframe comprises aplurality of long time slots and a plurality of short time slots. Eachof the long time slots comprises up to N OFDMA subcarrier transmitted bythe plurality of subscriber stations. The base station receives anacknowledgment signal transmitted by the first subscriber station. Theacknowledgment signal is transmitted on a first selected OFDMAsubcarrier in a first long time slot of the uplink subframe and on asecond selected OFDMA subcarrier in a second long time slot of theuplink subframe.

Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, itmay be advantageous to set forth definitions of certain words andphrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like. Itshould be noted that the functionality associated with any particularcontroller may be centralized or distributed, whether locally orremotely. Definitions for certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 illustrates an exemplary wireless network that transmits ACK/NACKmessages in the uplink according to the principles of the presentdisclosure;

FIG. 2A is a high-level diagram of an OFDMA transmitter according to oneembodiment of the present disclosure;

FIG. 2B is a high-level diagram of an OFDMA receiver according to oneembodiment of the present disclosure;

FIG. 3 is a message flow diagram illustrating hybrid ARQ messagesaccording to one embodiment of the disclosure;

FIG. 4 illustrates an uplink subframe according to one embodiment of theprior art;

FIG. 5 illustrates an uplink subframe according to one embodiment of thepresent disclosure;

FIG. 6 illustrates an uplink subframe according to another embodiment ofthe present disclosure;

FIG. 7 illustrates an uplink subframe according to another embodiment ofthe present disclosure;

FIG. 8 illustrates an uplink subframe according to another embodiment ofthe present disclosure;

FIG. 9 illustrates an uplink subframe according to another embodiment ofthe present disclosure;

FIG. 10 illustrates an uplink subframe according to another embodimentof the present disclosure;

FIG. 11 illustrates an uplink subframe according to another embodimentof the present disclosure;

FIG. 12 illustrates an uplink subframe according to another embodimentof the present disclosure; and

FIG. 13 illustrates selected portions of an exemplary base station andan exemplary subscriber station according to another embodiment of thepresent disclosure.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 13, discussed herein, and the various embodiments usedto describe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged communication system.

The present disclosure describes a frequency-multiplexing approach forthe ACK/NACK channel in an OFDM or DFT-spread OFDM system. The disclosedfrequency-multiplexing approach enables ACK/NACK signal transmissionover a longer duration and therefore allows the ACK/NACK signal to carrymore energy. The higher transmitted energy on the ACK/NACK channelimproves ACK/NACK signal reception and thereby improves system coverageand throughput.

The transmission technique disclosed herein may advantageously beembodied in a wireless network that dynamically allocates resources tohybrid Acknowledgement Request (ARQ) messages according to thedisclosure in U.S. patent application Ser. No. 11/390,056, incorporatedby reference above. Thus, a resource (e.g., communication channel)allocated to an ACK/NACK message may be dynamically identified in acontrol channel message accompanying the data packet or data subpackettransmission from the transmitting device (e.g., a base station). Thereceiving device (e.g., a subscriber station) then sends an ACK or NACKmessage informing the transmitting device about the successful orunsuccessful transmission of the packet. The ACK/NACK is sent using theresource identified in the control channel message sent by thetransmitting device.

Moreover, the disclosed transmission technique may be embodied in awireless network that use Fourier Transform pre-coding to reduce thepeak-to-average power (PAPR) ratio according to the disclosure in U.S.patent application Ser. No. 11/374,928, entitled “Apparatus And MethodFor FT Pre-Coding Of Data To Reduce PAPR In A Multi-Carrier WirelessNetwork” and filed on Mar. 14, 2006. Application Ser. No. 11/374,928 isassigned to the assignee of this application and is hereby incorporatedby reference as if fully set forth herein.

In the descriptions that follow, it shall be assumed generally thattransmitters and receivers are operating in OFDMA mode. However, thisembodiment should not be construed to limit the scope of the disclosure.In alternate embodiments, the transmitters and receivers may operate inOFDM mode or another multi-carrier mode without departing from theprinciples of the disclosure.

FIG. 1 illustrates exemplary wireless network 100, which transmitsACK/NACK messages according to the principles of the present disclosure.In the illustrated embodiment, wireless network 100 includes basestation (BS) 101, base station (BS) 102, base station (BS) 103, andother similar base stations (not shown). Base station 101 is incommunication with base station 102 and base station 103. Base station101 is also in communication with Internet 130 or a similar IP-basednetwork (not shown).

Base station 102 provides wireless broadband access (via base station101) to Internet 130 to a first plurality of subscriber stations withincoverage area 120 of base station 102. The first plurality of subscriberstations includes subscriber station 111, which may be located in asmall business (SB), subscriber station 112, which may be located in anenterprise (E), subscriber station 113, which may be located in a WiFihotspot (HS), subscriber station 114, which may be located in a firstresidence (R), subscriber station 115, which may be located in a secondresidence (R), and subscriber station 116, which may be a mobile device(M), such as a cell phone, a wireless laptop, a wireless PDA, or thelike.

Base station 103 provides wireless broadband access (via base station101) to Internet 130 to a second plurality of subscriber stations withincoverage area 125 of base station 103. The second plurality ofsubscriber stations includes subscriber station 115 and subscriberstation 116. In an exemplary embodiment, base stations 101-103 maycommunicate with each other and with subscriber stations 111-116 usingOFDM or OFDMA techniques.

Base station 101 may be in communication with either a greater number ora lesser number of base stations. Furthermore, while only six subscriberstations are depicted in FIG. 1, it is understood that wireless network100 may provide wireless broadband access to additional subscriberstations. It is noted that subscriber station 115 and subscriber station116 are located on the edges of both coverage area 120 and coverage area125. Subscriber station 115 and subscriber station 116 each communicatewith both base station 102 and base station 103 and may be said to beoperating in handoff mode, as known to those of skill in the art.

Subscriber stations 111-116 may access voice, data, video, videoconferencing, and/or other broadband services via Internet 130. In anexemplary embodiment, one or more of subscriber stations 111-116 may beassociated with an access point (AP) of a WiFi WLAN. Subscriber station116 may be any of a number of mobile devices, including awireless-enabled laptop computer, personal data assistant, notebook,handheld device, or other wireless-enabled device. Subscriber stations114 and 115 may be, for example, a wireless-enabled personal computer(PC), a laptop computer, a gateway, or another device.

FIG. 2A is a high-level diagram of an orthogonal frequency divisionmultiple access (OFDMA) transmit path. FIG. 2B is a high-level diagramof an orthogonal frequency division multiple access (OFDMA) receivepath. In FIGS. 2A and 2B, the OFDMA transmit path is implemented in basestation (BS) 102 and the OFDMA receive path is implemented in subscriberstation (SS) 116 for the purposes of illustration and explanation only.However, it will be understood by those skilled in the art that theOFDMA receive path may also be implemented in BS 102 and the OFDMAtransmit path may be implemented in SS 116.

The transmit path in BS 102 comprises channel coding and modulationblock 205, serial-to-parallel (S-to-P) block 210, Size N Inverse FastFourier Transform (IFFT) block 215, parallel-to-serial (P-to-S) block220, add cyclic prefix block 225, up-converter (UC) 230, and maincontroller and scheduler 235 (hereafter, simply main controller 235).The receive path in SS 116 comprises down-converter (DC) 255, removecyclic prefix block 260, serial-to-parallel (S-to-P) block 265, Size NFast Fourier Transform (FFT) block 270, parallel-to-serial (P-to-S)block 275, channel decoding and demodulation block 280, and maincontroller 285.

At least some of the components in FIGS. 2A and 2B may be implemented insoftware while other components may be implemented by configurablehardware or a mixture of software and configurable hardware. Inparticular, it is noted that the FFT blocks and the IFFT blocksdescribed in this disclosure document may be implemented as configurablesoftware algorithms, where the value of Size N may be modified accordingto the implementation.

Furthermore, although this disclosure is directed to an embodiment thatimplements the Fast Fourier Transform and the Inverse Fast FourierTransform, this is by way of illustration only and should not beconstrued to limit the scope of the disclosure. It will be appreciatedthat in an alternate embodiment of the disclosure, the Fast FourierTransform functions and the Inverse Fast Fourier Transform functions mayeasily be replaced by Discrete Fourier Transform (DFT) functions andInverse Discrete Fourier Transform (IDFT) functions, respectively. Itwill be appreciated that for DFT and IDFT functions, the value of the Nvariable may be any integer number (i.e., 1, 2, 3, 4, etc.), while forFFT and IFFT functions, the value of the N variable may be any integernumber that is a power of two (i.e., 1, 2, 4, 8, 16, etc.).

In BS 102, channel coding and modulation block 205 receives a set ofinformation bits, applies coding (e.g., Turbo coding) and modulates(e.g., QPSK, QAM) the input bits to produce a sequence offrequency-domain modulation symbols. Serial-to-parallel block 210converts (i.e., de-multiplexes) the serial modulated symbols to paralleldata to produce N parallel symbol streams where N is the IFFT/FFT sizeused in BS 102 and SS 116. Size N IFFT block 215 then performs an IFFToperation on the N parallel symbol streams to produce time-domain outputsignals. Parallel-to-serial block 220 converts (i.e., multiplexes) theparallel time-domain output symbols from Size N IFFT block 215 toproduce a serial time-domain signal. Add cyclic prefix block 225 theninserts a cyclic prefix to the time-domain signal. Finally, up-converter230 modulates (i.e., up-converts) the output of add cyclic prefix block225 to RF frequency for transmission via a wireless channel. The signalmay also be filtered at baseband before conversion to RF frequency.

The transmitted RF signal arrives at SS 116 after passing through thewireless channel and reverse operations to those at BS 102 areperformed. Down-converter 255 down-converts the received signal tobaseband frequency and remove cyclic prefix block 260 removes the cyclicprefix to produce the serial time-domain baseband signal.Serial-to-parallel block 265 converts the time-domain baseband signal toparallel time domain signals. Size N FFT block 270 then performs an FFTalgorithm to produce N parallel frequency-domain signals.Parallel-to-serial block 275 converts the parallel frequency-domainsignals to a sequence of modulated data symbols. Channel decoding anddemodulation block 280 demodulates and then decodes the modulatedsymbols to recover the original input data stream.

The transmit path and receive path components described herein andillustrated in FIGS. 2A and 2B are configurable devices that may bere-programmed and controlled by main controller 235 in BS 102 or maincontroller 285 in SS 116. Thus, for example, main controller 235 isoperable to configure modulation block 205 to adapt to differentmodulation techniques (e.g., BPSK, QPSK, QAM, etc.). Similarly, maincontroller 285 is operable to similarly configure demodulation block280. Main controllers 235 and 285 are also operable to modify the valueof Size N.

Each of base stations 101-103 may implement a transmit path that isanalogous to transmitter 200 for transmitting in the downlink tosubscriber stations 111-116 and may implement a receive path that isanalogous to receiver 250 for receiving in the uplink from subscriberstations 111-116. Similarly, each one of subscriber stations 111-116 mayimplement a transmit path corresponding to the architecture oftransmitter 200 for transmitting in the uplink to base stations 101-103and may implement a receive path corresponding to the architecture ofreceiver 250 for receiving in the downlink from base stations 101-103.

There may be a large number of subscriber stations present in wirelessnetwork 100. Due to the bursty nature of data traffic, typically only afew subscriber stations are scheduled to receive a transmission at agiven time. A mechanism for dynamic allocation of ACK/NACK channels wasdisclosed in U.S. patent application Ser. No. 11/390,056, which wasincorporated by reference above. Main controllers 235 and 285 areoperable to allocate uplink channel resources to subscriber stations111-116 as described in U.S. patent application Ser. No. 11/390,056. Inone embodiment of the present disclosure, each one of base stations101-103 is capable of dynamically allocating uplink channel resources tosubscriber stations 111-116 according to the number of subscriberstations that will be receiving downlink data transmissions and will berequired therefore to send ACK or NACK messages (and associated pilotsignals) back to a transmitting base station. The uplink channelresources may be independently and selectively allocated for eachtransmission, rather than being permanently dedicated to particularsubscriber stations.

FIG. 3 depicts message flow diagram 300, which illustrates hybrid ARQmessages according to an exemplary embodiment of the present disclosure.Base station (BS) 102 transmits control channel message 305 tosubscriber station (SS) 116 at the same time that BS 102 transmits datamessage 310. Control channel message 305 contains ACK/NACK ResourceIndication (1), which indicates or identifies the uplink channelresources (i.e., subcarriers and time slots) that SS 116 is to use totransmit back an ACK message or a NACK message and pilot signals. Datamessage 310 contains Subpacket 1 of Packet A. Assuming Subpacket 1 ofPacket A is not properly decoded, SS 116 responds by transmitting NACKmessage 315 using the uplink channel resources indicated in message 305for sending ACK messages and NACK messages.

BS 102 then transmits control channel message 320 to SS 116 at the sametime that BS 102 transmits data message 325. Control channel message 320contains ACK/NACK Resource Indication (2), which indicates or identifiesthe uplink channel resources (i.e., subcarriers and time slots) that SS116 is to use to transmit an ACK message or a NACK message and pilotsignals. ACK/NACK Resource Indication (2) in message 320 may be the sameas ACK/NACK Resource Indication (1) in message 305, or it may bedifferent.

If wireless network 100 implements a hybrid ARQ protocol, data message325 contains Subpacket 2 of Packet A. SS 116 will combine Subpacket 1and Subpacket 2 in order to attempt to decode Packet A. Assuming SS 116is able to decode Packet A from Subpacket 1 and Subpacket 2, SS 116responds by transmitting ACK message 330 using the uplink channelresources indicated in message 320 for sending ACK messages and NACKmessages.

By way of example, if wireless network 100 uses turbo coding in thedownlink, Subpacket 1 may comprise i) the systematic data applied to theturbo encoder, and ii) a first portion of the parity data generated bythe turbo encoder. Subpacket 2 may comprise the remaining portion of theparity data generated by the turbo decoder. If the systematic bits andthe first portion of parity bits in Subpacket 1 cannot be used torecover all of Packet A, then the turbo decoder in SS 116 combines theremaining portion of parity bits in Subpacket 2 with the systematic bitsand the first portion of parity bits in Subpacket 1 to recover Packet A.

In an exemplary embodiment, the ACK/NACK feedback may be a two-statesignal (i.e., +1 for ACK, −1 for NACK) that is transmitted as a datafield within a larger feedback message. Alternatively, the ACK/NACKfeedback may be three-state signal (i.e., +1 for ACK, −1 for NACK, or 0for DTX). In the three-state ACK/NACK feedback, the receiver also needsto differentiate the no transmission (DTX) state from ACK and NACKstate. Therefore, for the same bit error (BER) performance, the energyrequired for three-state feedback is larger than two-state feedback.

FIG. 4 illustrates an uplink subframe according to one embodiment of theprior art. The uplink subframe transmits control information and userdata from the subscriber station to the base station. The exemplarysubframe has a length of 0.5 milliseconds (msec.) and is divided into aplurality of time slots. The time slots comprise six long blocks(LB1-LB6) and two short blocks (SB1 and SB2). Each one of long blocksLB1-LB2 has a length of 66.6 microseconds (usec.). Each one of shortblocks SB1 and SB2 has a length of 33.3 microseconds. A cyclic prefix(CP) is added to each of LB1-LB6 and SB1 and SB2.

In a conventional system, short blocks SB1 and SB2 carry the pilotsignal for channel estimation while long blocks LB2-LB6 carry the datasymbols. However, conventional wireless networks uses long block LB1 tocarry the entire ACK/NACK signal. For a given transmit power of P wattsand a time of T=66.6 microseconds, the total transmitted energy for theACK/NACK signal in long block LB1 is only P×T Joules.

FIG. 5 illustrates uplink frame 500 according to one embodiment of thepresent disclosure. As in the prior art example in FIG. 4, uplink frame500 has a length of 0.5 milliseconds and comprises a plurality of timeslots, namely long blocks (LBs) and short blocks (SBs). In particular,uplink frame 500 comprises six long blocks (LB1-LB6) and two shortblocks (SB1 and SB2). Each one of long blocks LB1-LB2 has a length of66.6 microseconds. Each one of short blocks SB1 and SB2 has a length of33.3 microseconds. A cyclic prefix (CP) is added to each of LB1-LB6, SB1and SB2.

In the example in FIG. 5, each symbol in long blocks LB1-LB6 comprises Nsubcarriers (or tones). The 512 subcarriers are labeled f1 through fN.In an exemplary embodiment, N=512 subcarriers and the subcarrier spacingin long blocks LB1-LB6 is 15 KHz. Each symbol in short blocks SB1 andSB2 comprises N/2 subcarriers (or tones) and the subcarrier spacing inshort blocks SB1 and SB2 is 30 KHz. Each subcarrier in short blocks SB1and SB2 occupies the same spectrum as two subcarriers in one of longblocks LB1-LB6. Thus, the first subcarrier in short block SB1 or SB2occupies the same spectrum as subcarriers f1 and f2 in long blocksLB1-LB6, the second subcarrier in short block SB1 or SB2 occupies thesame spectrum as subcarriers f3 and f4 in long blocks LB1-LB6, the thirdsubcarrier in short block SB1 or SB2 occupies the same spectrum assubcarriers f5 and f6 in long blocks LB1-LB6, and so forth. Finally, thelast (or N/2) subcarrier in short block SB1 or SB2 occupies the samespectrum as subcarriers f(N−1) and fN in long blocks LB1-LB6.

According to the principles of the present disclosure, the uplinkACK/NACK channel structure uses a frequency-multiplexing approach. InFIG. 5, the ACK/NACK channels are carried on a pair of subcarriers. Afirst ACK/NACK signal (or ACK1) is transmitted on subcarrier fA. Asecond ACK/NACK signal (or ACK2) is transmitted on subcarrier fB. Eachone of the ACK1 signal and the ACK2 signal comprises six symbols, withone symbol per long block (LB) and each long block time slot having aduration T=66.6 microseconds. By way of example, the ACK1 signal iscarried on the fA subcarrier in each one of long blocks LB1-LB6 and theACK2 signal is carried on the fB subcarrier in each one of long blocksLB1-LB6. Thus, the total energy that can be carried for a given transmitpower P is 6×P×T Joules.

The ACK1 signal carried over the fA subcarriers uses the pilot signal P1transmitted in short block SB1, while the ACK2 signal carried over thefB subcarriers uses the pilot signal P2 transmitted in short block SB2.Pilot signal P1 uses subcarriers fA and fB in short block SB1 and pilotsignal P2 uses subcarriers fA and fB in short block SB2. Since the pilotsignals P1 and P2 use the same subcarriers as the ACK1 and ACK 2signals, reliable channel estimates may be obtained for demodulation ofthe ACK1 and ACK 2 signals.

FIG. 6 illustrates uplink frame 600 according to another embodiment ofthe present disclosure. In FIG. 6, additional pilot P1 and P2 symbolsfor each of the ACK1 and ACK2 channels are also carried in two longblocks (i.e., LB3 and LB4) in addition to the pilot P1 and P2 symbolscarried in short blocks SB1 and SB2. In this embodiment, a total of 4symbols are used for each ACK signal and 3 symbols for each pilotsignal. The additional pilot symbols allow for better channel estimationfor demodulation of the ACK signals.

FIG. 7 illustrates uplink frame 700 according to another embodiment ofthe present disclosure. In FIG. 7, pilot signals are carried on thesubcarrier adjacent to the ACK/NACK signal subcarrier over the six longblocks. By way of example, the ACK1 signal is carried on subcarrier fAin each of long blocks LB1-LB6 and the corresponding pilot signal P1 iscarried on subcarrier fB adjacent to subcarrier fA. Similarly, the ACK2and pilot P2 signals are carried on adjacent subcarriers fC and fD ineach of long blocks LB1-LB6.

FIG. 8 illustrates uplink frame 800 according to another embodiment ofthe present disclosure. In FIG. 8, pilot signals are carried on thesubcarrier adjacent to the ACK/NACK signal subcarrier over the six longblocks. In addition, symbols for the two pilot signals P1 and P2 arealso carried on the fA, fB, fC and fD subcarriers in short blocks SB1and SB2.

FIG. 9 illustrates uplink frame 900 according to another embodiment ofthe present disclosure. FIG. 9 is similar in most respects to FIG. 8,except that pilot signals P1 and P2 are carried on the subcarriersadjacent to the subcarriers for the ACK1 and ACK2 signal in only 5 ofthe long blocks, namely LB1, LB2, LB4, LB5 and LB6. However, in longblock LB5, both pairs of adjacent subcarriers are used for the ACK1 andACK2 signals. Thus, subcarriers fA and fB carry the ACK1 signal, whilesubcarriers fC and fD carry the ACK2 signal. This configuration allowsfor equal distribution of symbols between ACK signals and pilot signals(i.e., 7 symbols for each ACK signal and 7 symbols for each pilotsignal).

FIG. 10 illustrates uplink frame 1000 according to another embodiment ofthe present disclosure. In FIG. 10, the ACK1 and ACK2 signals areuniformly distributed over the whole used bandwidth. This enableswireless network 100 to exploit frequency diversity. As in FIGS. 7-9,the symbols for the pilot signals P1 and P2 are transmitted insubcarriers adjacent to the subcarriers for the ACK1 and ACK2 signals toaid the channel estimation in demodulating the ACK/NACK signals. In FIG.10, the ACK1 signal and the pilot signal P1 use subcarriers fA and fB inlong block LB1, subcarriers fC and fD in long block LB2, subcarriers fEand fF in long block LB3, and so forth. Similarly, the ACK2 signal andthe pilot signal P2 use subcarriers fC and fD in long block LB1,subcarriers fE and fF in long block LB1, subcarriers fG and fH in longblock LB3, and so forth. The ACK2 signal and the pilot signal P2 usesubcarriers fA and fB in long block LB6.

FIG. 11 illustrates uplink frame 1100 according to another embodiment ofthe present disclosure. In FIG. 11, the subcarriers used for theACK/NACK and pilot signals are distributed at the two edges of the usedbandwidth. This configuration provides some frequency diversity for theACK signal while leaving contiguous bandwidth in the middle for othercontrol and data channels.

By way of example, subcarrier f1 is used by the ACK1 signal in longblocks LB2 and LB6, the ACK2 signal in long blocks LB1 and LB5, thepilot signal P1 in long block LB4, and the pilot signal P2 in long blockLB3. Also, in short block SB2, the first 30 KHz subcarrier carries thepilot signal P2 using the same spectrum as subcarriers f1 and f2.Similarly, subcarrier fN is used by the ACK1 signal in long blocks LB1and LB3, the ACK2 signal in long blocks LB2 and LB6, the pilot signal P1in long block LB5, and the pilot signal P2 in long block LB4. Also, inshort block SB1, the last 30 KHz subcarrier carries the pilot signal P1using the same spectrum as subcarriers f(N−1) and fN.

FIG. 12 illustrates uplink frame 1200 according to another embodiment ofthe present disclosure. In FIG. 12, as in FIG. 11, the ACK/NACK andpilot signals are again distributed at the two edges of the usedbandwidth. However, the pilot signals are transmitted in subcarriersadjacent to the ACK/NACK subcarriers in long blocks LB1-LB6. In stillanother embodiment of the disclosure, the subcarriers at the edges ofthe used bandwidth may be used by a single ACK/NACK signal. By way ofexample, the ACK1 signal and the pilot signal P1 may be transmitted insubcarriers f1, f2, f(N−1) and fN in long blocks LB1-LB6.

FIG. 13 illustrates selected portions of exemplary base 102 station andexemplary subscriber station 116 according to another embodiment of thepresent disclosure. A DFT-spread OFDM system is attractive for theuplink (i.e., subscriber station to base station) of a wireless systemdue to its low peak-to-average power (PAPR) characteristic. Thiscompensates for the limited transmit power available in a subscriberstation. A low PAPR enables a lower power amplifier back off and allowsthe subscriber station to transmit at a higher power and higher datarate, thereby improving the coverage and spectral efficiency of awireless network.

In FIG. 13, subscriber station (SS) 116 comprises size M FFT block 1310,ACK/NACK amplifier 1320, pilot amplifier 1330, M data amplifiers,including amplifiers 1340 a and 1340 b, and size N IFFT block 1350. Basestation (BS) 102 comprises size N FFT block 1360, frequency domainequalization (FDE) block 1370, and size M IFFT block 1380. DFT-spreadOFDM systems similar to FIG. 13 were disclosed in U.S. patentapplication Ser. No. 11/374,928, which was incorporated by referenceabove.

In a DFT-spread OFDM system, the coded modulation data symbols are FFTpre-coded by size M FFT clock 1310 before mapping at the input of size NIFFT block 1350. The FFT pre-coded outputs of size M IFFT block 1310 maythen be scaled by a gain factor, g3, by the M amplifiers 1340. ACK/NACKamplifier 1320 applies a gain factor, g1, to the ACK/NACK signals andpilot amplifier 1330 applies a gain factor, g2, to the pilot signals. Inan exemplary embodiment of the disclosure, gain factors g1, g2 and g3may have different values.

At BS 102, size N FFT block 1360 recovers the data symbols and the pilotsymbols. FDE block 1370 performs frequency-domain equalization after theFFT operation. Size M IFFT block 1380 then performs an IFFT operation onthe equalized symbols in order to obtain the data modulation symbolsthat were FFT pre-coded in SS 116.

The frequency-multiplexed approach for the ACK/NACK signals, the pilotsignals, and the user data allows for different gain factors and powerallocations to these different signals. This provides the ability tocontrol the reliability of the ACK/NACK channel according to the desiredbit error rate (BER) requirement.

In FIGS. 1-12 above, it has generally been assumed that the ACK1 and ACK2 signals provided only a single-bit ACK/NACK feedback. However, theprinciples of present disclosure also apply when multi-bit ACK/NACKfeedback is required, such as in MIMO multi-codeword embodiment. In thiscase, it is also possible to perform channel coding on multiple ACK/NACKbits before mapping to subcarriers/symbols.

Although the present disclosure has been described with an exemplaryembodiment, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present disclosure encompasssuch changes and modifications as fall within the scope of the appendedclaims.

What is claimed is:
 1. For use in a wireless network, a base stationthat communicates with a plurality of subscriber stations according to amulticarrier protocol, wherein the base station is configured to receivean uplink subframe comprising a plurality of blocks, each of the blockscomprising up to N subcarriers transmitted by the plurality ofsubscriber stations, and receive, from the plurality of subscriberstations, acknowledgement (ACK) signals carried on subcarriers that aresubstantially at edges of a bandwidth of the N subcarriers such that ina first block of the subframe, a first ACK signal is carried on a firstsubcarrier that is substantially at a first edge of the bandwidth whilea second ACK signal is carried on a subcarrier that is substantially ona second edge of the bandwidth, and such that in a second block of thesubframe, the first ACK signal is carried on the second subcarrier whilethe second ACK signal is carried on the first subcarrier.
 2. The basestation as set forth in claim 1, wherein the plurality of blockscomprises six long blocks and two short blocks, and each ACK signal iscarried in at least four long blocks.
 3. The base station as set forthin claim 1, wherein the base station is further configured to receive afirst pilot signal associated with the first ACK signal and a secondpilot signal associated with the second ACK signal.
 4. The base stationas set forth in claim 3, wherein the first pilot signal is carried on atleast the first subcarrier in a first short block, and the second pilotsignal is carried on at least the second subcarrier in a second shortblock.
 5. The base station as set forth in claim 3, wherein each pilotsignal is carried on at least one subcarrier that is adjacent to thecorresponding ACK signal in at least one long block.
 6. The base stationas set forth in claim 1, wherein each ACK signal is not received on asame carrier in consecutive long blocks.
 7. The base station as setforth in claim 1, wherein the base station is further configured toreceive data between the first and second subcarriers.
 8. Acommunication device that transmits a first message in a forward channelto at least one other communication device and receives multicarriersignals in a reverse channel from the other communication devices, thereverse channel comprising a plurality of transmission slots, each ofthe transmission slots comprising up to N subcarriers transmitted by theother communication devices, and receive, from the at least one othercommunication device, acknowledgement (ACK) signals carried onsubcarriers that are substantially at edges of a bandwidth of the Nsubcarriers such that in a first transmission slot of the reversechannel, a first ACK signal is carried on a first subcarrier that issubstantially at a first edge of the bandwidth while a second ACK signalis carried on a subcarrier that is substantially on a second edge of thebandwidth, and such that in a second transmission slot of the reversechannel, the first ACK signal is carried on the second subcarrier whilethe second ACK signal is carried on the first subcarrier.
 9. Thecommunication device as set forth in claim 8, wherein the plurality oftransmission slots comprises six long transmission slots and two shorttransmission slots, and each ACK signal is carried in at least four longtransmission slots.
 10. The communication device as set forth in claim8, wherein the communication device is further configured to receive afirst pilot signal associated with the first ACK signal and a secondpilot signal associated with the second ACK signal.
 11. Thecommunication device as set forth in claim 10, wherein the first pilotsignal is carried on at least the first subcarrier in a first shorttransmission slot, and the second pilot signal is carried on at leastthe second subcarrier in a second short transmission slot.
 12. Thecommunication device as set forth in claim 10, wherein each pilot signalis carried on at least one subcarrier that is adjacent to thecorresponding ACK signal in at least one long transmission slot.
 13. Thecommunication device as set forth in claim 8, wherein each ACK signal isnot received on a same carrier in consecutive long transmission slots.14. The communication device as set forth in claim 8, wherein thecommunication device is further configured to receive data between thefirst and second subcarriers.
 15. For use in a wireless network thatcommunicates with a plurality of subscriber stations according to anorthogonal frequency multiple access (OFDMA) protocol, a base stationconfigured to transmit a message to from at least one subscriber stationin a downlink channel, receive in an uplink channel an uplink subframecomprising OFDMA subcarriers transmitted by the plurality of subscriberstations, wherein the uplink subframe comprises a plurality of long timeslots and a plurality of short time slots, each of the long time slotscomprising up to N OFDMA subcarriers transmitted by the plurality ofsubscriber stations, and receive, from the at least one subscriberstation, acknowledgement (ACK) signals carried on subcarriers that aresubstantially at edges of a bandwidth of the N subcarriers such that ina first long time slot of the uplink subframe, a first ACK signal iscarried on a first subcarrier that is substantially at a first edge ofthe bandwidth while a second ACK signal is carried on a subcarrier thatis substantially on a second edge of the bandwidth, and such that in asecond long time slot of the uplink subframe, the first ACK signal iscarried on the second subcarrier while the second ACK signal is carriedon the first subcarrier.
 16. The base station as set forth in claim 15,wherein each ACK signal is carried in at least four long time slots. 17.The base station as set forth in claim 15, wherein the base station isfurther configured to receive a first pilot signal associated with thefirst ACK signal and a second pilot signal associated with the secondACK signal.
 18. The base station as set forth in claim 17, wherein thefirst pilot signal is carried on at least the first subcarrier in afirst short time slot, and the second pilot signal is carried on atleast the second subcarrier in a second short time slot.
 19. The basestation as set forth in claim 17, wherein each pilot signal is carriedon at least one subcarrier that is adjacent to the corresponding ACKsignal in at least one long time slot.
 20. The base station as set forthin claim 15, wherein each ACK signal is not received on a same carrierin consecutive long time slots.
 21. The base station as set forth inclaim 15, wherein the base station is further configured to receive databetween the first and second subcarriers.
 22. For use in wirelessnetwork capable of communicating with a plurality of subscriber stationsaccording to a multicarrier protocol, a method of a base stationcommunicating with selected ones of the subscriber stations, the methodcomprising: transmitting a first message in a downlink channel from thebase station to at least one subscriber station; receiving by the basestation an uplink subframe comprising a plurality of blocks, each of theblocks comprising up to N subcarriers transmitted by the plurality ofsubscriber stations; and receiving, from the at least one subscriberstation, acknowledgement (ACK) signals carried on subcarriers that aresubstantially at edges of a bandwidth of the N subcarriers such that ina first block of the uplink subframe, a first ACK signal is carried on afirst subcarrier that is substantially at a first edge of the bandwidthwhile a second ACK signal is carried on a subcarrier that issubstantially on a second edge of the bandwidth, and such that in asecond block of the uplink subframe, the first ACK signal is carried onthe second subcarrier while the second ACK signal is carried on thefirst subcarrier.
 23. The method as set forth in claim 22, wherein theplurality of blocks comprises six long blocks and two short blocks, andeach ACK signal is carried in at least four long blocks.
 24. The methodas set forth in claim 22, further comprising receiving a first pilotsignal associated with the first ACK signal and a second pilot signalassociated with the second ACK signal.
 25. A method of communicatingbetween a communication device and at least one other communicationdevice according to a multicarrier protocol, the method comprising:transmitting, by the communication device, a first message in a forwardchannel to a the at least one other communication device; receiving, bythe communication device, multicarrier signals in a reverse channel froma plurality of other communication devices, the reverse channelcomprising a plurality of transmission slots, each of the transmissionslots comprising up to N subcarriers transmitted by the othercommunication devices; and receiving, from the at least one othercommunication device, acknowledgement (ACK) signals carried onsubcarriers that are substantially at edges of a bandwidth of the Nsubcarriers such that in a first transmission slot of the reversechannel, a first ACK signal is carried on a first subcarrier that issubstantially at a first edge of the bandwidth while a second ACK signalis carried on a subcarrier that is substantially on a second edge of thebandwidth, and such that in a second transmission slot of the reversechannel, the first ACK signal is carried on the second subcarrier whilethe second ACK signal is carried on the first subcarrier.
 26. The methodof communicating as set forth in claim 25, wherein the plurality oftransmission blocks comprises six long transmission blocks and two shorttransmission blocks, and each ACK signal is carried in at least fourlong transmission blocks.
 27. The method of communicating as set forthin claim 25, further comprising receiving a first pilot signalassociated with the first ACK signal and a second pilot signalassociated with the second ACK signal.
 28. For use in a base station ofa wireless network capable of communicating with a plurality ofsubscriber stations according to an orthogonal frequency multiple access(OFDMA) protocol, a method of communicating with selected ones of thesubscriber stations, the method comprising: transmitting a message fromthe base station to the selected ones of the subscriber stations in adownlink channel; receiving by the base station an uplink subframecomprising OFDMA subcarriers transmitted by the plurality of subscriberstations, wherein the uplink subframe comprises a plurality of long timeslots and a plurality of short time slots, each of the long time slotscomprising up to N OFDMA subcarrier transmitted by the plurality ofsubscriber stations; and receiving, from the selected ones of thesubscriber stations, acknowledgement (ACK) signals carried onsubcarriers that are substantially at edges of a bandwidth of the Nsubcarriers such that in a first long time slot of the uplink subframe,a first ACK signal is carried on a first subcarrier that issubstantially at a first edge of the bandwidth while a second ACK signalis carried on a subcarrier that is substantially on a second edge of thebandwidth, and such that in a second long time slot of the uplinksubframe, the first ACK signal is carried on the second subcarrier whilethe second ACK signal is carried on the first subcarrier.
 29. The methodas set forth in claim 28, wherein each ACK signal is carried in at leastfour long time slots.
 30. The method as set forth in claim 28, furthercomprising receiving a first pilot signal associated with the first ACKsignal and a second pilot signal associated with the second ACK signal.31. A subscriber station for use in wireless network capable ofcommunicating according to a multicarrier protocol, wherein thesubscriber station is configured to receive from a first base station ina downlink channel a first message, and in response to receipt of thefirst message, transmit an acknowledgment (ACK) signal to the first basestation in an uplink subframe comprising a plurality of blocks, whereinthe subscriber station transmits a first part of the ACK signal on afirst subcarrier in a first block of the uplink subframe and transmits asecond part of the ACK signal on a second subcarrier in a second blockof the uplink subframe, the first subcarrier is substantially at a firstedge of a bandwidth of the block, and the second subcarrier issubstantially at a second edge of the bandwidth.
 32. The subscriberstation as set forth in claim 31, wherein the plurality of blockscomprises six long blocks and two short blocks, and each ACK signal iscarried in at least four long blocks.
 33. The subscriber station as setforth in claim 31, wherein the subscriber station is further configuredto transmit a pilot signal associated with the ACK signal on the uplinksubframe.
 34. The subscriber station as set forth in claim 33, whereinthe pilot signal is carried on at least the first subcarrier in a firstshort block.
 35. The subscriber station as set forth in claim 31,wherein the pilot signal is carried on at least one subcarrier that isadjacent to the ACK signal in at least one long block.
 36. Thesubscriber station as set forth in claim 31, wherein the subscriberstation is further configured to transmit data between the first andsecond subcarriers.